Many CMOS and BiCMOS ICs comprise a large digital core and some analog peripheral functions. These analog functions typically include reference circuits used, among other things, for the analog block, for supply voltage regulation, and for certain of the digital circuits (e.g., for power-on-reset circuits). The most widely used implementation of voltage reference circuits with a low temperature coefficient is the so-called bandgap-reference circuit.
Outputting a reference voltage close to the bandgap of the silicon of 1.205V, bandgap circuits have long been a standard for realization using either bipolar or CMOS transistors. Today, however, it is very difficult, or even impossible, to design a reference circuit that outputs bandgap voltage in most advanced CMOS technologies because for proper operation, the supply voltage Vdd of the bandgap circuit has to be higher than the bandgap voltage Vbg, usually 1.3-1.5V. On the other hand, the supply voltage of CMOS circuits has been continuously falling from 3.3V for 0.35 μm process, 2.5V for 0.25 μm, 1.8V to 0.18 μm, and 1V for today's 90 nm technology, respectively, as illustrated in FIG. 1. It is seen in this Figure that roughly from 0.13 μm CMOS technology onwards, the supply voltage Vdd becomes too low for any reference circuit to output the bandgap voltage Vbg=1.205V.
In general, the output voltage of most of the known CMOS and non-CMOS bandgap reference circuits is the sum of a diode voltage and the voltage across a resistor. Typically, the current that flows through the resistor is proportional to the absolute temperature in a way to compensate, in the first order, the negative temperature coefficient of the forward voltage of the diode.
This current can be generated in several manners. In a typical CMOS bandgap voltage reference circuit the current is generated in such a way that it is linearly dependent from the temperature and usually the thermic voltage Ut is used. If a bandgap voltage with higher accuracy over temperature is required, quite a complex curvature compensation has to be used. Furthermore, as mentioned above, this kind of bandgap reference circuit cannot be employed at supply voltages below the semiconductor material bandgap voltage.
In U.S. Pat. No. 6,566,850 B2, a low-voltage bandgap reference circuit is proposed. A current is simply mirrored into a transistor and flows to an output resistor. This circuit thus provides an output voltage which is not very stable over temperature. The shown circuit cannot be operated at very low voltages. It is another disadvantage of the circuit proposed in this US patent that a depletion type transistor is required. Adding such a depletion type transistor to a standard process entails additional costs.
U.S. Pat. No. 6,160,393 concerns a voltage reference circuit where a PTAT current is forced to flow through a combination of a pMOS transistor and an nMOS transistor in series or in parallel. The shown circuit cannot be operated at very low voltages. Furthermore, the temperature performance of the circuit is not addressed at all. The-temperature stability appears to be worse than the stability of a conventional bandgap. It is a further disadvantage of the circuit proposed in U.S. Pat. No. 6,160,393 that is requires a simultaneous ion implantation for the nMOS and pMOS transistors. This, however, is not available for standard CMOS processes.
Yet another circuit is presented in U.S. Pat. No. 6,680,643 B2. This circuit requires many different elements, such as a PTAT circuit, an operational transimpedance amplifier, a differential operational amplifier, an amplifier current extraction circuit and an output stage. It is obvious that this leads to a very complex realization. This complexity adds to the costs, make the development time longer, degrades the reliability and consumes more power. Furthermore, the supply voltage has to be at least 1.5 V.
There is, therefore, a strong demand for a new principle for generating the reference voltage in today's and future CMOS technologies where the reduced supply voltage does not pose any constraint to hamper the realization of a reference voltage. At such a low supply voltage Vdd, it is clear that the generated reference voltage has to be lower than the bandgap voltage in value.
Furthermore, it would be generally desirable to provide a solution allowing a reference voltage to be generated with equal or even better performance in temperature stability.
It is thus an object of the present invention to provide a reference voltage generator that can be employed even in situations where the supply voltage Vdd is lower than the bandgap voltage.
It is a further objective of the present invention to provide a reference voltage generator that providing a reference voltage that is less temperature dependent than the reference voltage provided by conventional bandgap reference circuits.
It is a further objective of the present invention to provide a reference voltage with very high accuracy and low-voltage.